Check for any exhaust leaks, if it has an exhaust leak, even a small one on the driverside manifold or anywhere else between the head and the left side o2 sensor, the leak not only blows exhaust out but also sucks air in causing the o2 on that bank to pick up the extra air which will set the bank 1 system lean code, also if you still have cats and did the 4.8 to 6.0 swap without the tune to correct for the extra cubic inches, the engine will run lean causing the cats to fail which will melt them, a melted or restricted right side cat can cause the left bank to run lean, no exhaust getting out means that the motor gets less fuel injector pulsewidth, but since the right cat could be restricted it would never set a lean code, only the left on would, those are some things you should check, if you need any extra info or diag let me know, I sure will be happy to be of some help....
Had a 99 GMC ESCB with LQ4 conversion and Nelson Performance tunning, currently workin on my 82 SCB GMC with carbed LQ4 conversion in progress, Also have a 00 SS Camaro that runs mid 11's all day long with bolt ons, Drag race my 79 malibu that has a 496 BBC runnin 10's on motor, shootin for 9's with the 250 shot of nitrous in its near future. Use my 04 2500 chevy to pull my rig around and drive the fam in our 07 NNBS yukon, Die Hard Chevy Fan Till The Death.